1. Field of the Invention
The present invention relates generally to structures for supporting offshore platforms above the surface of the water and more particularly to a structure that includes a leg having an overall cross-sectional area that is greatest adjacent the seabed and decreases moving upwardly along such leg from the seabed to a region above the seabed and is substantially constant moving from such region to the platform. While the invention has application in many types of offshore platform configurations, it is particularly useful in connection with mobile, jackup type offshore platforms.
2. Description of the Prior Art
Rigs for performing various operations at sea, often referred to as "offshore rigs," generally include a superstructure or platform supported above the surface of the water by a support structure extending between the superstructure and the seabed.
In the prior art, support structures have commonly included a plurality of legs extending either vertically or at an angle between the platform and the seabed. The legs of the prior art ordinarily have a substantially uniform cross-sectional area along their entire length. For examples of such prior art legs, see, e.g., U.S. Pat. No. 3,466,878, issued to Esquillan et al. on Sept. 16, 1969, (cylindrical or tubular legs) and U.S. Pat. No. 3,183,676, issued to LeTourneau on May 18, 1965 (lattice-type legs). The legs may engage the seabed by means of spud caissons (see, e.g., Le Tourneau, supra), or by means of piles driven into the seabed (see, e.g., Esquillan, supra), or by means of a flat surface, referred to as a "mat" interconnecting the lowermost ends of the legs and resting directly on the seabed (see, e.g., U.S. Pat. No. 3,699,688, issued to Estes on Oct. 24, 1979). The legs may be interconnected by a truss network as shown in U.S. Pat. No. 3,093,972, issued to Ward on June 18, 1963. The legs of the prior art sometimes do not have a uniform cross-sectional area along their entire length. For examples of such prior art legs, see, e.g., U.S. Pat. No. 4,045,968, issued to Gerwick, Jr. on Sept. 6, 1977. For examples of other non-uniform cross-section structures which might be placed in the water, see, e.g., U.S. Pat. No. 3,201,945, issued to Sutton on Aug. 24, 1965 and drawings of Offshore Equipment Development Co. bearing the date of July 12, 1976.
Generally speaking, the support structure, including the number, arrangement and actual configuration of the legs must be established so that the structure is capable of supporting several thousand tons of weight above the surface of the water with a high degree of overall stability as well as withstanding the often tremendous forces of winds, waves, currents and tides. As the length of the legs of the support structure increases, however, the potential of a leg of the support structure to fail due to the forces of wind, waves and/or currents increases. In order to lessen this potential for failure, there has been a tendency in the prior art to increase the structural strength of longer length legs by increasing the cross-sectional area of such legs uniformly along their entire length or to add frame-type bracing extending their entire length (see, U.S. Pat. No. 3,007,316, issued to Higgins, Jr. on Nov. 7, 1961). It is known in the prior art, however, that the force per unit length arising from wave motion acting on a submerged cylindrical pile can be approximated as: ##EQU1## where f=force per unit length=density of the fluid
.rho.U=velocity perpendicular to the pile due to wave motion PA0 .differential.U/.differential.t=acceleration perpendicular to the pile due to wave motion PA0 D=pile diameter PA0 C.sub.M =inertial coefficient PA0 C.sub.D =drag coefficient
In accordance with the foregoing, it can be seen that the force per unit length generated by a particular wave increases as the cross-sectional area of the pile increases. Thus, as the cross-sectional area of a leg of a support structure is increased for the purpose of withstanding the forces of waves, the effective force generated by a particular wave also increases. Similarly, additional bracing along the entire length of the leg adds to the total cross-sectional area of the leg, including the region along the leg where the velocity and acceleration of the fluid in a wave is the greatest, whereby the force generated by the wave at that region is increased. As a result, the prior art techniques of increasing the structural strength of the legs uniformly along their entire length is highly inefficient.